A Thin Film Transistor (TFT) Liquid Crystal Display (LCD) is one of Active Matrix Liquid Crystal Displays (AM-LCDs). So far, the TFT-LCD is the unique display device comparable with or superior to the Cathode Ray Tube (CRT) display device in terms of comprehensive performances including brightness, contrast, power consumption, service life, volume, weight and the like. TFT-LCDs have been increasingly widely used due to their excellent performances, and good characters for mass production, including high automation level and low costs of raw material.
A TFT-LCD touch panel is a device used for outputting two-dimensional image signals, like an image capturing device, a touch panel with charge integrating amplifiers is a device for outputting two-dimensional spatial touch signals, and sensing lines need to capture relatively small extraneous signals. Accordingly, it is essential to suppress various dark background noise including static and dynamic dark currents from the sensing lines of the touch panel.
Due to the relatively large capacitance of touch electrodes, the static electricity caused by a small amount of charges would not cause the breakdown of an insulating film, thus the presently available capacitive touch panels have not been provided with any ESD protection circuits. However, after the resolution of touch electrodes is increased significantly, the capacitance of each touch electrode becomes smaller. In the manufacturing process, the occurrence probability of an electrostatic damage increases with the enlargement of the substrate of the touch panel. Accordingly, an ESD protection circuit dedicated to the touch panel needs to be added to a TFT-LCD panel, and is connected to a sensing or driving electrode of the touch panel through the upper and lower electrical connection. As shown in FIG. 1, the ESD protection circuit used in the existing and most commonly used TFT-LCD, the ESD protection circuit is used for protecting data lines and scanning lines of the TFT-LCD from the breakdown caused by static electricity. Apparently, the ESD protection circuit can also be used to protect driving signal electrodes and sensing signal electrodes of the touch panel from the breakdown caused by static electricity.
The ESD protection circuit illustrated in FIG. 1, the ESD protection circuit includes two TFTs (T1 and T2) connected in a back-to-back manner, is connected between a ground line 12 and a sensing line or driving line 11.
FIG. 2 illustrates a graph showing both static leak current of the ESD protection circuit illustrated in FIG. 1 and voltage across of the ESD protection circuit illustrated in FIG. 1.
During the manufacturing of the product, the ESD protection circuit illustrated in FIG. 1 can be used to rapidly release both static charges accumulated on a glass substrate and static charges transferred from machines and environment, thus to protect the TFT-LCD touch panel. However, in the normal driving of the TFT-LCD touch panel, the static leak current of the ESD protection circuit would likely cause the saturation or malfunction of a pre-amplifier (such as a charge integrating amplifier) of the touch panel.
Accordingly, those skilled in the art need to solve the problem of an influence on the TFT-LCD touch panel caused by the static leak current of the ESD protection circuit.